Physicists have already unveiled invisibility cloaks that can hide objects from light, sound, seismic and even water waves. Now researchers report a cloak that can hide objects from static magnetic fields. This 'antimagnet' could have medical applications, but might also subvert airport security.


The cloak's interior is lined with turns of tape made from a high-temperature superconductor. Superconductors repel magnetic fields, so any magnetic field enclosed within a superconductor would be undetectable from outside. But the superconductor itself would still perturb an external magnetic field, so the researchers coated its external side with an ordinary ferromagnet — the material that kitchen fridge magnets are made of. The superconductor tries to repel external field lines, whereas the ferromagnet tries to draw them in — together, the two layers cancel each other out.

To test the antimagnet, the Slovak group cooled the cloak with liquid nitrogen to activate the superconductor, and placed it in a static, uniform magnetic field with a strength of 40 millitesla. Using a measuring device called a Hall probe to map the magnetic field, the researchers found that the field lines did not enter the cloak, even through from the outside they appeared to pass straight through. They say that theirs is an 'exact' cloak — one for which the cloaking could, in principle, be made perfect using currently available materials.

Sanchez points out that the magnetic cloak is straightforward to make: it requires only off-the-shelf materials and costs in the region of €1,000 (US$1,300) — very little in research terms. He believes the cloak could have uses in medicine, protecting delicate pacemakers from the strong magnetic fields of magnetic resonance imaging machines. But he admits there could also be unsavoury applications — the technology could, for example, be used to hide metallic weapons from security portals. “I would prefer to consider it the other way — that our ideas can help to design safer security procedures,” Sanchez says.

Taking the wraps off cloaking, John Pendry
The real challenge of cloaking lies in deriving a theoretical prescription for the optical properties of the cloak and, even more challenging, realizing these properties in a material. Transformation optics provides the theoretical background and metamaterials provide the means of achieving the prescribed parameters.

Transformation optics
It was Michael Faraday who stressed the importance of “lines of force.” He could see magnetic lines of force aligning iron filings placed near his magnets, and for him they represented physical reality. Lines of force are continuous and their density represents the strength of a field. Likewise, electric field lines are also continuous, at least in the absence of electrical charges. In fact, any conserved vector quantity can be represented in this way, and one can add the Poynting vector, representing the flow of electromagnetic energy, to this set. The Poynting vector is merely the mathematical representation of a “ray” of light.

Maxwell’s equations are a mathematical realization of Faraday’s work. They describe the phenomena of classical optics and it has long been known that their form is invariant under coordinate transformations.

Transformation optics was born of the realization that lines of force are effectively glued to the coordinate system. As the system is distorted it carries with it all the associated fields. Hence to guide the trajectory of a ray of light, only a distortion in the underlying coordinate system is needed, automatically taking with it the light ray. Knowledge of the transformation in turn provides the values of μ and ε required to steer the light in this way.

Invisibility
Figure 3 (top left) shows a ray of light travelling in free space. We wish to hide the contents of a sphere radius R1 by directing the rays around this region, but requiring that any distortion of trajectories is confined within a larger sphere radius R2 (Fig. 3, top right). In this way an external observer would be aware neither of the presence of the cloak nor its contents. The illusion of empty space has been created.

Metamaterials
The idea behind metamaterials is that electromagnetic properties can be altered by changing the physical structure of an existing material. A simple example would be to create a series of voids in a dielectric. If the voids are much smaller than the relevant wavelength, incident light will see only an average response and a reduced effective permittivity. More complex structures, such as tiny metallic resonators, can produce more exotic effects, including negative refractive indices. Provided that the structure remains much smaller than the wavelength, properties can be interpreted in terms of an effective ε and μ. As it happens, nature has not been generous with the electromagnetic properties of materials: only a limited range is on offer and some properties, such as a negative refraction, are entirely absent in naturally occurring materials. In fact, it was the ability of metamaterials to produce negative refractive indices that first drove them to prominence. Since that time, the concept of function through structure has been deployed to implement many theoretical schemes previously thought to be beyond reach.

This has been #Cloaking101 part 1 of #InterstellarTravel101